A display using a self-luminous thin film light-emitting element which emits light by being applied with current has been actively developed.
These light-emitting elements emit light by connecting an electrode to a single thin layer or multiple thin layers made from either or both of an organic material and an inorganic material and by applying current to the electrode. Such the thin film light-emitting element is a promising element in terms of low power consumption, a small footprint, visibility, or the like, and it is expected to bring about expansion of markets.
In the case of a light-emitting element having a multiple layered structure, functions of the layer are divided, consequently, a light-emitting element which emits light more efficiently than the conventional light-emitting element can be manufactured (for example, refer to non-patent document 1: C. W. Tang et al., “Applied Physics Letters”, Vol. 51, No. 12, pp. 913-915 (1987)).
A light-emitting element having a multiple layered structure is constructed by interposing a light emitting laminated body formed by a hole injecting layer, a hole transporting layer, a light-emitting layer, an electron transporting layer, an electron injecting layer, and the like between an anode and a cathode. Any of the hole injecting layer, the hole transporting layer, the electron transporting layer, and the electron injecting layer may not be used depending on an element configuration. The hole transporting layer and the electron transporting layer may serve as a light-emitting layer. In that case, a method for doping dye having high light emission efficiency to the electron transporting layer having a high carrier transporting property or the hole transporting layer is used. By using the method, a material which has high light emission efficiency but a low carrier transporting property can be used for forming the light-emitting layer (for example, refer to non-patent document 2: C. W. Tang et al., “Journal of Applied Physics”, Vol. 65, No. 9, pp. 3710-3716, (1989)).
The thin film light-emitting element having the foregoing characteristics is expected to be used as a lighting system. In the case that the thin film light-emitting element is used as a lighting system, it is desired that the thin film light-emitting element emits white light emission. Therefore, it is attempted that a light-emitting element is made to include a plurality kinds of luminescence centers exhibiting different emission colors to obtain white light emission (for example, refer to patent document 1: Japanese Patent Application Laid-Open No. H7-142169).
White emission is recognized by stimulating uniformly pyramidal cells of human eyes. To manufacture a light-emitting element which emits light recognized as white emission, it is known that light-emitting materials exhibiting three colors of red, green, and blue, respectively, can be emitted simultaneously, or two kinds of light-emitting materials which exhibit two complementary colors of emission can be emitted simultaneously.
However, white emission formed in such a way has often a spectrum having projecting shapes of each color, and is inferior in color rendering properties to natural light which has the same intensity in all wavelength components.
That is, the color of an object is recognized by human eyes by absorbing or reflecting light at a specified wavelength band. However, in the case that light at a specified wavelength band has extremely high intensity or extremely low intensity, the color seems to change by being specified light enhanced or tamed compared to light having the same intensity in all wavelength components such as sunlight.
In view of the foregoing, it is desired that emission spectra of the light-emitting element, the light-emitting device, and the lighting system are broad such as sunlight. It is an object of the present invention to provide such the light-emitting element, light-emitting device, and lighting system, each of which has high color rendering properties.